The present invention relates to a magnetic memory device, particularly to a solid state magnetic memory device which is suited to a miniature mass storage memory device.
In the recent information-oriented society, each individual handles an enormous amount of information. There is an increasing demand for high density solid state storage elements as memory devices as miniature information related equipment for processing such information. Magnetic bubble memory and Bloch line memory are well-known solid state magnetic memories. The Bloch line memory, as indicated in Japanese Patent Application Laid-Open No. 59-101092, has a feature that the memory density is higher by about 2 digits than that of the magnetic bubble memory because it stores information in a micro magnetization structure (Bloch line pair). When the memory density is increased, much information can be stored in a limited area, so that a memory device which is economical and easy to operate can be realized.
However, it is known that it is difficult to increase the memory density of the Bloch line memory device, which can be increased most, to more than 1 giga bits per cm.sup.2. It is due to the structure of the conventional Bloch line memory device. This problem will be described with reference to FIGS. 2a and 2b.
FIG. 2a is a plan view of the information storing region of the conventional Bloch line memory. The information storing region comprises magnetic garnet films and a pattern group for determining the information storing position, which is called Bit Pattern 4, formed on the film. Magnetic domain walls 3 (a magnetization transition region which exists on the boundary between the magnetic domains) are provided almost perpendicularly to the bit pattern 4, and information, which is set to 1 or 0 in correspondence with the presence or absence of a micro magnetization structure formed on each magnetic domain wall which is called a Bloch line pair 2 (schematically indicated by arrows in the same way as with conventional patents), is stored. The bit pattern 4 comprises a magnetic material and others, and has a function for determining the stable position of each Bloch line pair by the interaction of the pattern strain field and the Bloch line pair. FIG. 2b is a schematic diagram of this status. FIG. 2b shows the shape of a potential well formed by the the interaction of the strain field and the Bloch line pair. The potential well periodically determines the stable position of the Bloch line pair.
The Bloch line memory moves each Bloch line pair to a predetermined position for input or output of information, converts the existence of the Bloch line pair to the existence of a bubble domain which can be read, or reversely converts the existence of the bubble domain to the existence of a Bloch line pair. This shift (also called propagation) of the Bloch line pairs is important for realizing the memory operation.
The shift of the Bloch line pairs can be realized by the force (acted in the tangential direction of each magnetic domain wall) generated by externally applying a pulsed magnetic field. By adjusting this force and the force for determining the Bloch line pair position, the Bloch line pair can be shifted to the position determination region of the neighboring bit pattern only when a pulsed magnetic field is externally applied. By doing this, the propagation function necessary of the memory operation can be realized.
The conventional Bloch line memory requires a bit pattern 4 which has the shape shown in FIG. 2a because it stores information. Since the bit pattern 4 comprises continuous simple rectangular patterns, it has been considered that the bit pattern 4 can be easily formed by the so-called lithographic technique using the optical means. As the memory density increases, however, the bit pattern size decreases to less than 1 micron and it becomes difficult to form the bit pattern 4 due to the optical resolution limit. To solve this problem, the X-ray lithography or the electron beam lithography which has a high resolution is required. The X-ray lithography or the electron beam lithography generally requires expensive equipment. Therefore, it is difficult to install such equipment so as to reduce the device production cost. Accordingly, the conventional method produces only solid state magnetic memories with a memory density of 10 Mb per cm.sup.2 using bit patterns of more than 1 micron.